Question #425ea

1 Answer
Jan 18, 2017

The primary kinetic isotope effect does not affect the reaction mechanism; rather, it gives information on the reaction mechanism.

Explanation:

Kinetic Isotope Effect

The kinetic isotope effect (KIE) refers to the change in rate caused by an isotopic substation in a molecule.

Frequently, the KIE refers to the effect of substituting "D" for "H" (the k_"H"//k_"D" ratio).

The "C-H" and "C-D" vibrations are quantized per the condition

E = (n + 1/2)hf, where n = 0, 1, 2, …

At ordinary temperatures, most of the molecules are in the n = 0 state.

We see the maximum KIE when a "C-H" bond is being broken in the transition state, because the vibrational mode disappears.

Since E_0("H") > E_0("D"), E_"a""(H") < E_"a""(D)" and k_"H" > k_"d".

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This is called a primary kinetic isotope effect.

The calculated value of k_"H" // k_"d" ≈ 7.

A secondary KIE occurs when the isotope is substituted at a position next to the bond being broken.

Usually, for a secondary KIE, k_"H"//k_"D" < 1.5.

Mechanistic information from KIEs

KIEs give useful information about the rate determining step in a mechanism.

Case 1

"CH"_3"CH"_2"CH"_2"Br" stackrelcolor(blue)( "EtO"^"-", "EtOH"color(white)(m)) (→) "CH"_3"CH=CH"_2

"CH"_3"CD"_2"CH"_2"Br" stackrelcolor(blue)( "EtO"^"-", "EtOH"color(white)(m)) (→) "CH"_3"CD=CH"_2

k_"H"//k_"D" = 6.7

This is consistent with an "E2" elimination in which the "C-H/D" bond is being broken in the rate determining step.

Case 2

"CH"_3"CH"_2"C"("CH"_3)_2"Br" stackrelcolor(blue)( "H"_2"O", Δcolor(white)(m)) (→) "CH"_3"CH=C"("CH"_3)_2

"CH"_3"CD"_2"C"("CH"_3)_2"Br" stackrelcolor(blue)( "H"_2"O", Δcolor(white)(m)) (→) "CH"_3"CD= C"("CH"_3)_2

k_"H"//k_"D" = 1.4

This is consistent with an "E1" elimination in which the "C-H(D)" bond is not being broken in the rate determining step.